Zinc salts of aromatic mercaptans as processing agents for rubbers



Patented Apr. 19, 1949 znsmss zinc-sumsor-momrro'ncnonrrmsAsmocEssmG.mews-Fort RUBBERS John J. Verb'anc, Tuxedo Bark, Del.,-assignor to 'E. I.-du1 l 'o'nt de Nemours & Company, Wilmington; DL,accorporationof Delaware No-Drawing; ApplicatioiflNovember 16,1944,

SerialNo. 5.633805 'filaliiims. 1 This invention relatesv .to the,processing of ,el'astomeric niateri'alsand toragents tor improvingtheiproces'sability .of elastomers, particularly those offthe'butadieneestyrene. type and )natural rubber;

The fact that-thejbutadienmstyrenelnterpoiymerssuch as the governmentrubber .know'nas GR 'S" do not brealt' down well. on continuedmastication, made the conversion from gth'e iiSe ofnatura'l'nrutber'.t'6' .tii'e Synthetic products extremely difd'cult. Theflackofjplasticizatiomof the GR-S rublci'ersv on milling causes a;material increase in the overalrmiliin time, for" the compoundingingredients must be added. very slowly in order tl'iattliey bedispcrsed' properly'in the ei'astome'r and'fto prevent the compoundedelastomer' "from becomin'g'laey and fallingoif gtiie mill. As a result,the outputof fiiiishediproduct from thissynth'etic' el'astomer perunit'goftimeis materiall decreased witn respect to the former output ofcompounded rubber fromtlie samerm'achinery.

While "a number of'substancesnave' been proposed assoitenersandpl'asticizers fornatural-an'd synthetic rubbers, most of thesubstances used heretofore, when applied to-the interpolymers ofbutadiene-sty'ren*e, do not give' the desired improvement in milling andprocessing characteristicseven though they may improve theplasticityzof-th'epproduct.

:In' copendingirapplication Serial No. 504,761, nowv U. S. 1?.2,378,519, .a. .methodis idisclosednfor improving: the processability ofelastomers by intimately incorporating-in the elastomerazino salt O'ffan aromatic mercaptan' of the benzene and naphthalene series. In thatapplication it is pointed: out that :not -zonlyi,-the plasticity :isimgenerally correlated.

-Itzis ancobiectzofatiie presentxi-nventionqtowpro= Vi'de processingregents: which; :when s-zadded in smalltamountsztozelastomeriesmaterials;andvmore particularly to those of therbutadieneestyrene typ will materially decrease the time required ionmillingv such elastomer and will also greatly decrease the overall timerequired for the additionwof the compounding agents thereto. A. furtherobjectof the invention is to provide aprocessingagent-which, when addedto the butadiene styrene elastomers,'willtincrease their plasticitysuiiiciently ate-impart theretoebetter molding propcr tics and.extrusion characteristics. A still furthereobjcct ofethe invention is toproduce a processingagent which will actat relatively low temperaturesso that the synthetic elastomers may beuworkedonithe usual design ofrubber-working eq ipment,

have found: that, by the addition of from 0.05% ito' 5%lof acompositionconsisting of a zinc saltof an aromatic ,mercaptan of the -benzene qornaphthalene series dissolved in an equal weight of amorganic solvent, oracorresponding amounteota solution of other-concentration, to the rawelastomers suchas the butadiene-styrene interpolymer, the milling andprocessing characteristics, as well as the plasticity of the resulting:elastomen. are materially improved. The processing ai -mastication ofthe elastomer inthe presenceor this processingagent maybe carried out inthe standard rubber processing: equipment suchsas the :Gordonplasticator, 'Banbury mixer .or on thecbnventional-rubberlmill.

Bmdissolvingihezinc salt of thearomatic mercaptancin tan'organicsolvent,the activity of the Zinnia-salt is materially increased. This increasedeiiectiveness cannot be explained'merely on the basisofuthe lgsolventacting za's a diluent, .for the solventsusuallyxvaporizes during themilling and doesnot remain .in the polymer; Furthermore, thez-a-mountioisolvent employed is so small that noiappreoiable softening action can beexpected f-rom itlalone.

zThevsolutionsaof the-zinc salts of the" aromatic mercaptansimayberprep'ared either by. dissolvingithexd-ry mercaptideinthe solvent orby reacting zin-oioxid'e With'thedesired mercaptan in the solventgasmoretparticu'larly described in my copending application Serial No.563,806, now U. S. Patent 2,413,531, issued December 31, 1946. he-eifect1on mil-labi-lity, plasticity, etc, is the samezinieither' case..The solutions "of the zinc mercapt'ides may contaimfrom- 5% to 75-%of-the 60% are preferred, with a solution being 3 most convenientlyused. The amount of the dissolved zinc mercaptide to be employed,irrespective of the amount of solvent in which it is dissolved, ispreferably between 0.05 to 2.5%, based on the weight of the elastomer,the preferred amount being between 0.25% and 2.0%.

Any of the usual methods employed for the incorporation of compoundingagents into elastomers may be used for introducing the solutions of thezinc mercaptides into the elastomers. They may be added on either a hotor cold mill or in a hot or cold Gordon plasticator or Banbury mixer.Any of these methods of mixing will produce a plastic mass into whichcompounding ingredients may be incorporated with ease.

The solvents which may be employed are the neutral oxygenated organicsolvents or mixtures of the same, such as alcohols, ketones,ketoalcohols, cyclic ethers, aliphatic ethers or mixtures of theseoxygenated solvents with hydrocarbon solvents such as xylene, benzene,gasoline, kerosene, hexane, etc. The mixtures may vary considerablywithout seriously aifecting the stability of the solution or theplasticizing action of the zinc aryl mercaptide dissolved therein, Ithas been found possible to vary the composition of such mixtures fromparts by Weight of the oxygenated solvent with 90 parts of a hydrocarbonsolvent to 90 parts of oxygenated solvent with 10 parts of hydrocarbonsolvent. The preferred solvent mixtures of the oxygenated andhydrocarbon solvents are those which contain from to '75 parts of theoxygenated solvent.

Typical of the zinc aryl mercaptides which Show improved properties asprocessing agents when dissolved in various oxygenated organic solventsis the zinc xylyl mercaptide. The effect of adding the organic solventsolution of this mercaptide to a standard sample of GR-S(butadiene-styrene interpolymer) is shown in the following Table I. Thedecreases in milling time caused by incorporation of the solution of thezinc xylyl mercaptide is taken as a measure of improvement inmillability in this class of elastomer. The milling time as reported inthis table and elsewhere in this description is determined as follows:

A 50 gram sample of the elastomer is placed on a laboratory mill havingrolls 6 inches long and 2 inches in diameter, both running at the samespeed, separated 0.030 inch. These rolls are kept at a temperature of 70C. throughout the test. The band on the mill roll is cut a definitenumber of times at stated intervals of time, to insure thorough mixing,for example, it is often convenient to cut the band four times each wayat 5 minute intervals during the test, The milling is continued untilthe band is free of holes for at least the time required for onerevolution of the front mill roll. The time in minutes elapsing betweenthe start of milling and the development of a band free of holes, istaken as a measure of millability. When a chemical agent is to be testedas a processing agent, such agent is added immediately after theelastomer is placed on the mill, and addition is accomplished as quicklyas possible. The conditions of this test diifer from actual plantconditions only in the size of equipment used. Throughout thisspecification the term millability refers to the milling time, inminutes, required in this test.

The solutions of zinc xylyl mercaptide used in these tests were preparedby heating equal weights of the desired solvent and the metal salt at toC. with good agitation and in such a manner that practically no solventwas lost due to vaporization. The resulting clear solutions were thenevaluated as processing agents.

TABLE I Solutions of zinc xylyl mercaptide as millability improaers ofGR-S 1 D z isi SPclrcent \l 11 b l x. lnc er 0 ution J. i a i ity No.solvent Employed eaptide in Based on at 70 0.

Solution Elastomer None None None 00. 0 do 4 23.5 Isobutanol 50 4 14. 5Isopropan 50 4 .24. 5 Lorol 50 i 18. 5 betaimethoxy etha- 50 1 22.5

no 7 Cellosolve 50 4 14.5 8 Methyl-ethyl Ke- 50 4 10. 0

tone. 0 Diacetone Alcohol 50 4 1G. 5 10...." Cyclohexanol 50 4 17.0 11.Mesityl Oxide 50 4 22.0 12 Dioxanc 50 4 16.5 13 Terpineol... 50 4 13.514 Isofenohonc. 50 4 14.0

1 GR-s (an interpolymer of butadiene and styrene prepared bypolymerization of approximately 75 parts of butadiene and 25 partsstyrene.)

2 Dry powder.

As illustrated in Table I, the zinc xylyl mercaptide, when used insolution, is considerably more eifective in improving the millability ofGR-S than when used alone. In nearly every case 2 parts of zinc xylylmercaptide in solution gave superior results to 4 parts when used alone.

The liquid in each case dispersed completely and uniformly throughoutthe batch of polymer and appeared to do so regardless of the speed ofaddition. This is of extreme importance in using GR-S, since mostchemicals disperse poorly, causing the resulting vulcanizate to haveinferior properties.

In order to show the applicability of our processing compositions invarious types of butadiene-styrene interpolymers, and the advantages tobe gained over the use of the pure zinc salt, a series of tests werecarried out employing samples of GR-S produced by several companiesassociated With the United States Government's Synthetic Rubber Program.The milling test used was the same as that previously described. Themillability data obtained is listed in Table II.

TABLE III Millability in minutes at 7 0 C.

- 47 of a 507 5 lsgpropano l Manufacturer Blank Mercagtide Solution oi(1007) Zinc Xylyl Mercaptide A 60 23. 5 24. 5 B 65 24. 0 17. 0 C 90 70.0 75. 0 D 45. O 30. O

recorded in Table III.

vactivegngedizent. 4% of.zinc .xylylmercaptide wasem- "*A s illustrated"in Table III, the solutions o'f 'v' rriou's I zinc 'me'reaptidesareallextremely elfectiveas processing agents for GR S as indicated y the"improvement in milling time. "Ex-amimeanf the plas'tic'izedpolymerindicates that the liquid material disperses throughout the iwhole'mass'and-exhibits a maximum} plasticizing errace-probably because "of bettercontact. It has also 'been" foundthat 'the liquid products maybe i'added'very'mapidlyif desired; with noloss in either the ;p1asticizingefiector in the physical properties 'of"the "resultant 'vulcanizates.How- 'everjm the 'case of the pure :solidit has been 'found that toorapida'dditi'on' results in poor "dispersion. The poordispersion'produces localized r'sipotsrof ipl'asticization'which aredetrimental in that "vulcanizates produced from such a plastirciaed.stock are 'non un'iform" in physical characitfer'i'stics.

".Thefefficacy of a 50% solution ofzinc xylyl "mereaptide", preparedaccording to tlieprocess of Example 1 :of mycopending application SerialNQY563806I1OW U. S. Patenti2',413,53,1,isslledDe- "camber 31, 19 46,1reacting zinc oxidewith xylyl' :mjerc'aptan'in a mixed solvent.coniposedflo'fLfiQ t na 1115101 xylene an'dl50 parts of isopropanol,was "tested on a3!) inchirubber mill, using,GR-Sprepared'byjManufacturer...A. .The mill-r011 temperature was.freg'ulated ,at 170 C. with a .niill. set"- ting .or i0j;040;mh. 'Theplasticity recoyeryidata listed in Table iv was obtained Ion i a"Wil1iar'ns jpariallel }p1a;te ,plastometer which (measures the i'thiclzness Lin thousandths .of an .inch of. a pellet iafter i'ial-minutesfdeformation at .80,-C. under -.a :Ioadof5000 grams. The, pellet: usedhas .a (volume of 2; cc. .and,' -an originalthicknessof 0.4 inch.LTheLrecoveryis .measured at room, temperature one minuteaftermemovingethelweightjfrom-,the pellet, ..and.lis the difference inthousandths of an inch between the recovered thickness of the pellet andthe,yplasticity. :;number. (Williams, Ind. Eng. Chem., 16, 362 (1924)TABLE IV Williams plasticity-recovery measurements 10 lbs. GR ;S"'on a30 laboratory mill 1 Part-of a 50%.solution 'zinc 'xylyl mercaptide insolvent 1 NoPlasticizer Plasticity, Recovery 1 and 2 represent checkdeterminations. l Solvent composed of 50 parts xylene and 50 partsisopropanol.

'Millability'ofGR-"s 1 1 I ":solvent Millability f' .Employed- 1 ,at 70C.

,l l one iNone -00 T "Z1110 -xyl yl mercaptide 23 .113 1.110.." L .25 1o 30 c 'thi -alpha-'naphtholate t incthioebeta-naphtholate. 29. 55

Zinc thio-phenolate 23 V. .llheccomnarisonssin Table sshow *that'ithe'::organic solvent: solution oiizlrrcz-xylylzmercaptide, einiadditionitozbeing a yeny e'fllcient processing agent as indicatedbyimprovedzmillabilitysottthe polymers, is also a poweri ul plasticingagent, as indicated by the Williams plasticity-recovery data.*s'dftening effect is beneficial'in azthat 'iit decreass the timenecessary for comp'ounding and produces stocks which extrude andmoldbetter. The use dfsolutions ofizincaryl mercaptides makes possible'the "elimination 'of the relatively large-volumes of oils, tars coaltar softeners, and thellike, which heretofore fhave been used in .largequantities-iniGR S .to obtain a soft, extrudable stock. The ,use ,oflargetquantitiesrof said materials are often'harmful to the physicalproperties of the resulting vulcanizate, particularly tensiles'trengthytear strength and resistance toflex-crackin'g.

Plasticity data: on 2 other solutions of zinc xylyl .mercaptide inwhichzmixed solvents wereused are listed inTable Vi. 1n.,these.tests,.,1%.of plasticizing agent (solid or .-so1ution),iwasusedaomGR-S-manuf-actured by prod'ucer -A The GR+$ was 2 milled on a. lab oratory,rubber-mill-havingmollgflz inches long and 'fi' i-nches in diameter,using 400 grams of elastomer per batch, and with'a mill rolltemperatureof C. Each solution consists of "equal weights of zincxylylmercaptideandmixed solvent. The combinationsof solvents are "shown in *Table "V. Itwill be seen that 015 zinc xy-lyl -mercaptide' in solutionq issubstantially equal"to 1% of the S611 zinc xy-lyl'mercaptide.

sel

Wzlliams plasticity recovery-measurements 1 ""Timeof 1 Milling in M 9Solvent Combination Plasticity; iRecoyery (.BureZinc xylyl .Mercaptide50% Lorb],'-50%'-Xylene Q 50% Dioxane, 50% Xylene. t;

5.0% =Methyl Cellosolve, 50%

Xylene;

7 from 16 to 24 hours at room temperature, and the plasticity-recoverydata was obtained on a Williams parallel plate plastometer. The dataobtained is listed in Table VI.

TABLE VI Williams plasticity-recovery measurements 400 grams naturalrubber on a 6 x 12" laboratory mill 1 and 2 are check determinations.

1 EPA No. 3=a 36.5% solution of xylyl mercaptan in an inert hydrocarbonsolvent.

A 50% solution of zinc xylyl mercaptide in a mixed solvent composed of50 parts of isopropanol and 50 parts of xylene.

The above data (Table VI) indicate that a solution of zinc xylylmercaptide is as efiective as xylyl mercaptan in plasticizing naturalrubber.

The aromatic solvent solutions particularly useful for the process ofthis invention are those containing zinc salts of aromatic mercaptans ofthe benzene and naphthalene series which contain no further functionalgroup. The zinc salts of the simple substituted aryl mercaptans, such asthose more particularly described in my copending application Serial No.563,806, may be employed.

While the above examples more particularly illustrate the advantagesobtained by the use of these new processing agents in the working ofsynthetic rubber of the GR-S type and in natural rubber, they may alsobe used to plasticize other types of artificial rubber such as the butylrubbers (GR-I; isobutylene-conjugated diene hydrocarbon interpolymers),and the butadiene acrylonitrile interpolymers, etc.

The use of the solutions of this invention makes possible the break downof polymeric materials, e. g., GR-S and natural rubber, more completelyand in a shorter period of time with an appreciable saving in powerconsumption and, consequently, a much greater output from each piece ofrubber compounding equipment. The solutions of various zinc mercaptideshave been shown to be considerably more effective in their plasticizingactions on polymeric materials than the pure solid. The materialsoutlined in this invention record disperse in elastomers much morereadily than the solid mercaptides and provide, in addition to improvedmillability, considerable softening, which is extremely beneficial sinceit materially aids in dispersing additional compounding agents.

Polymeric materials plasticized with the compositions of this inventionalso dissolve more readily in organic solvents forming cements having alower viscosity.

I claim:

1. The process for improving the processing characteristics ofelastomers of the class consisting of natural rubber and butadienepolymer rubbers which comprises intimately incorporating in saidelastomer by mixing operation from 0.25% to 2.5%, based on the weight ofthe clastomer, of a zinc salt of an aromatic mercaptan selected from thegroup consisting of mercaptans of the benzene and naphthalene serieswhich is in solution in an inert organic solvent, the solvent consistingof from 10% to of a neutral oxygenated organic solvent.

2. The process for improving the processing characteristics ofbutadiene-styrene elastomers which comprises intimately incorporating insaid elastomer by mixing operation from 0.25% to 2.5%, based on theweight of the elastomer, of a zinc salt of an aromatic mercaptanselected from the group consisting of mercaptans of the benzene andnaphthalene series which is in solution in an inert organic solvent, thesolvent consisting of from 10% to 100% of a neutral oxygenated organicsolvent.

3. The process for improving the processing characteristics ofbutadiene-styrene elastomers which comprises intimately incorporating insaid elastomer by mixing operation from 0.25% to 2.5%, based on theweight of the elastomer, of the zinc salt oi xylyl mercaptan which is insolution in an inert organic solvent, the solvent consisting of from 10%to 100% of a neutral oxygenated organic solvent.

4. The process for improving the processing characteristics ofbutadiene-styrene elastomers which comprises intimately incorporating insaid elastomer by a mixing operation from 0.25% to 2.5%, based on theweight of the elastomer, of a zinc salt of a mercaptan of the benzeneseries which is in solution in an inert organic solvent consisting offrom 10% to 100% of an alcohol.

5. The process for improving the processing characteristics ofbutadiene-styrene elastomers which comprises intimately incorporating insaid elastomer by a mixing operation from 0.25% to 2.5%, based on theweight of the elastomer, of a zinc salt of a mercaptan of the benzeneseries which is in solution in an inert organic solvent consisting offrom 10% to 100% of isopropanol.

6. The process for improving the processing characteristics ofbutadiene-styrene elastomers which comprises intimately incorporating insaid elastomer by a mixing operation from 0.25% to 2.5%, based on theweight of the elastomer, of the zinc salt of xylyl mercaptan dissolvedin an inert organic solvent, which solvent consists of from 10% to 100%of isopropanol.

JOHN J, VERBAN C REFERENCES CITED The following references are of recordin the fizc of this patent:

UNITED STATES PATENTS Number Name Date 2,378,519 Vincent June 19, 19452,413,531 Verbanc Dec. 31, 1946 FOREIGN PATENTS Number Country Date390,532 Great Britain Apr. 7, 1933 428,456 Great Britain May 7, 1935526,131 Great Britain Sept. 11, 1940

